It is well known to use steam traps to retain steam in a system, while permitting condensate to be removed. One specific type of steam trap is the inverted bucket-type steam trap which has been widely used for controlling the flow of condensate from a steam heated unit to a condensate return line. Such traps are generally used to permit disposal of the condensate while at the same time minimizing the loss of live steam from the steam heated unit. U.S. Pat. No. 4,149,557, assigned to the assignee of the present invention, discloses a successful steam trap of the aforementioned kind.
A problem in prior art inverted bucket steam traps may occur during operation under very low condensate loads. The problem is illustrated with respect to prior art FIG. 6. FIG. 6 shows a prior art inverted bucket steam trap in very simplified schematic form.
Before discussing the aforementioned problem, attention is directed to normal operation of inverted bucket steam traps of which the FIG. 6 prior art trap is an example.
In general, a steam trap 10 (FIG. 6) is typically installed in a drain line 11 between a steam heated unit HU and a condensate return header RH of a steam system. The steam system normally includes a heat source H for converting water to steam and applying it to the steam heated unit HU. The steam heated unit HU extracts heat energy from the steam, reducing a portion of the steam to condensate. The condensate is to be returned to the heat source for reheating to steam. Thus, the steam heated unit HU outputs condensate, normally with some entrained uncondensed steam, applying same on the drain line 11 to the steam trap 10 which is intended to remove the condensate and route it back to the heat source through the condensate return header RH while minimizing loss of steam from the steam heated unit HU.
The mixed steam and condensate is applied to the trap 10 through an inlet conduit 12. The steam trap 10 has a casing 13. An inverted bucket 14 is loosely received in the casing 13 and can rise and fall therein as hereafter described. The inlet conduit 12 has a portion 15 extending into the casing and extending upward into the open bottom of the inverted bucket 14. A restrictive orifice, or vent, 16 is provided in the top of the bucket 14 and allows gradual escape of steam from the top portion of the bucket to the top portion of the casing 10. A valve member 17 is pivoted with respect to the casing 13 and is raisable to close a valve seat 20 to close the path from the steam trap 10 to the condensate return header RH. The valve member 17 is connected to, and is closed and opened by, the inverted bucket 14 as it rises and falls within the casing 11.
In normal operation, when the inverted bucket is in its lower most position (below that shown in FIG. 6), the valve 17 is in its lower (open) position not shown, leaving the upper portion of the casing 13 open to the condensate return header RH.
An initial flood of condensate enters the trap 10 through the inlet conduit 12 and flows beneath the lip of the inverted bucket 14 radially outward to fill the casing 13 and completely submerge the inverted bucket 14. Excess condensate is discharged past the open valve member 17 and seat 20 to the condensate return header RH.
When steam enters the trap from the drain line 11 and inlet conduit 12, it collects in the top portion of the inverted bucket 14, imparting buoyancy thereto. Inverted bucket 14 will then rise and lift the valve member 17 toward its seat 20. When the valve member 17 is close to the seat 20, but is still spaced therefrom by a small distance, a further flow of condensate past the valve member 17 and seat 20 toward the condensate return header RH will effect a snapping of the valve member 17 into its closed position shown in FIG. 6. When the valve member 17 closes the seat 20, any air and non-condensable gas trapped in the upper portion of the inverted bucket will gradually pass through the vent 16 therein and collect at the top of the casing 13. Similarly, steam which reaches the upper end of the bucket 14 will gradually flow through the vent 16, such flow being at a slow controlled rate. This steam is eventually condensed by radiation of heat from the steam trap.
Condensate continues to flow into the underside of the bucket through the extension 15 and thence into the trap casing 12. When the condensate level in the steam trap casing 13 reaches a level which is slightly above the floating level for the inverted bucket 14, the inverted bucket will exert a slight pull downward on the valve member 17. However, the valve member 17 will not be moved down to its open position until the condensate level rises to a predefined opening line in the unit for the existing pressure differential between the steam and the pressure in the condensate return header RH. When the condensate reaches this level, the weight of the inverted bucket multiplied by the leverage achieved by the length of its valve member 17 exceeds the pressure holding the valve member 17 in the seating engagement with the valve seat 20. The inverted bucket 14 will then sink and open the valve 17,20 to thereby allow excess condensate to discharge from the casing 13.
However, as above mentioned, a problem occurs when the inverted bucket trap 10 is subjected to very low condensate loads. Under this condition, steam flow is usually impeded by the accumulation of condensate in the low portion of the conduit 12 leading up into the inverted bucket 14. This prevents steam from freely entering the steam trap. In this condition, steam in the inlet conduit 12 tends to push condensate ahead of it in slugs into the steam trap. Such "slug flow" of condensate upsets the water level under the bucket, causing it to be unstable. However, it is such water level under the bucket that controls the opening and closing of the steam trap valve 17,20. As a result, valve operation tends to be unstable, with more rapid than normal opening and closing of the valve 17,20 and a tendency toward incomplete valve opening. This in turn can lead to premature wearing out of the valve 17,20 and premature failure of, and need to replace, the steam trap. It is through recognition of these problems that the present invention arises.
Accordingly, the objects and purposes of the invention include provision of a steam trap capable of cycling effectively on very low condensate loads, in which steam is allowed to enter the trap unimpeded by condensate flow, in which steam moving toward the trap does not tend to push condensate ahead of it in slugs, in which slug flow and consequent upset of the water level under the bucket and resulting instability of the level of the bucket are avoided, and in which these benefits are achieved with relatively little extra structure or cost as compared to the total cost of a conventional steam trap.